This invention relates to electrically operated control systems for gas burners in which a pilot burner is cycled on and off with the closing and opening of a space thermostat and gas flow to a main burner is effected by thermostatic means heated by pilot flame and, particularly, to a system of this kind in which the re-institution of gas flow to a main burner following the cut off thereof due to electrical power failure and prior to the assured re-establishment of an adequate pilot flame is precluded.
The U.S. Pat. No. 3,975,135 discloses a gas burner control system in which a normally closed, electromagnetically operated valve 76 controls gas flow to a pilot burner 14 and the application of gas pressure to the diaphragm chamber 46 of a normally closed diaphragm operated valve 38. The valve 76 opens and a spark igniter 16 becomes operative to ignite the pilot burner upon closure of a space thermostat 20. Intermediately of electromagnetic valve 76 and diaphragm chamber 46 is a thermostatically operated valve 74 having an expanding fluid-type thermostatic actuator including a bulb 100, which when sufficiently heated by pilot flame causes valve 74 to open and admit gas pressure to open valve 38.
In this arrangement, it will be seen that if the electrical power supply fails the electromagnetic valve 76 will instantly close, cutting off gas flow to pilot burner 14 and cutting off the application of gas pressure to diaphragm chamber 46. As a result, the flame at the pilot burner 14 and main burner 12 will be extinguished immediately. The diaphragm chamber 46 exhausts through orifice 82, permitting valve 12 to close rapidly. The thermostatically operated valve 74 will not, however, close immediately as a result of power failure. Some period of time is required for the fluid in bulb 100 and the chamber 98 to cool and contract or condense sufficiently, following extinguishing of the pilot flame, to permit the biasing spring 106 to close valve 74.
If the electrical power supply failure is only momentarily and is restored in a matter of a few seconds, which frequently occurs, the valve 74 will still be open when power is restored. As a result, valve 76 will open instantly upon restoration of power and main diaphragm valve 38 will open immediately, causing a large amount of gas to flow into the burner combustion chamber. At the same time, gas will flow to the pilot burner and the spark igniter will begin operation. The safe operation of gas burners, particularly the main burners of central heating systems provided with pilot burners, requires that an adequate pilot flame be established prior to the admission of gas flow to the main burner to insure instant ignition under any conditions which may occur in operation. Even a slight delay in the establishment of pilot flame when fuel is flowing from a main burner presents a potentially hazardous condition in systems designed to operate with a pilot burner.
An object of the present invention is to provide an improved gas burner control system employing a cycling pilot burner with thermostatic means heated by pilot burner flame for controlling gas flow to a main burner, in which means is provided to insure the establishment of adequate pilot flame under all conditions of operation prior to admission of gas flow to the main burner.
Further objects and advantages will appear from the following description when read in connection with the accompanying drawing.